One of the best features of a CMS like Joomla is that you can easily change the entire look of a website without changing any of the content that has been created. A brand new look is often only a matter of applying a new default Template and changing Module positions in the Joomla Administrator.
Joomla Templates can be purchased from a commercial developer, or you can create them yourself. It is also possible to work in a hybrid way by purchasing a basic Template that suits most of your needs, and then modifying it to acheieve the final touches you're looking for.
It's also possible to employ a Joomla Template designer in creating a custom Template to your exact specifications.